US4742568A

Movatterモバイル変換

Info

Publication number: US4742568A
Application number: US06/872,502
Authority: US
Inventors: Yukitsuna Furuya
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1985-06-10
Filing date: 1986-06-10
Publication date: 1988-05-03
Anticipated expiration: 2006-06-10
Also published as: GB8614104D0; GB2178269B; GB2178269A

Abstract

A receiver comprising a switching circuit responsive to an antenna switching signal for switching between two antennas to produce a received signal, a noise estimating circuit for estimating the quantity of noise from the received signal, and an integrating circuit for integrating the noise to produce a first control signal which serves as the antenna switching signal.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a receiver for use in antenna switching diversity systems.

In radio communication, there is experienced a phenomenon known as fading, wherein electric field strength of received signals (referred to "field strength") is varied by the influence of radio wave reflections by buildings and hills, sometimes so intensely that wave reception is made totally impossible. One of the known techniques to cope with fading is called diversity reception, in which two antennas are provided so that reception can be achieved with one of them even if impossible with the other. Switching diversity systems, whose structure is simpler because only the antenna has to be duplicated but the receiver circuit does not, are especially effective for mobile communications, where equipment has to be particularly small. A prior art example of the switching diversity system is described in detail in Microwave Mobile Communications, 1974, John Wiley & Sons, pp. 399-401. The system described in the publication, however, involves the problem that antenna switching does not take place when the field strengths for both antennas become lower than the threshold.

SUMMARY OF THE INVENTION

An object of the present invention, therefore, is to provide a receiver for use in switching diversity systems, which overcomes the aforementioned disadvantage and can switch the antenna even when the field strength is low.

According to the invention, a receiver for use in switching diversity systems comprises a switching circuit responsive to a switching signal for switching between two antennas to produce a received signal. The receiver further comprises a detector for determining the amount of noise in the received signal and an integrator for accumlating the amount of noise and supplying the switching signal to the switching circuit when the accumulated value has surpassed a certain limit.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram illustrating a first preferred embodiment of the invention;

FIGS. 2 and 3 are circuit diagrams illustrating a power detector;

FIG. 4.illustrates an example of an integrator;

FIGS. 5A to 5C are diagrams for explaining the operation of the receiver of FIG. 1;

FIG. 6 is a block diagram illustrating a second preferred embodiment of the invention;

FIGS. 7A to 7C are diagrams for explaining the operation of the second preferred embodiment;

FIG. 8 is a block diagram illustrating a third preferred embodiment of the invention;

FIGS. 9 and 10 illustrate a comparator and a memory, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram illustrating a first preferred embodiment of the invention. Referring to the figure, anantenna switching circuit 10 switches between twoantennas 1 and 2 in response to a switching signal from anintegrator 30, which will be described below. A signal received by one of the antennas is supplied to apower detector 20, which estimates the power of noise in the received signal and feeds the estimated noise power to theintegrator 30. Theintegrator 30 integrates the noise power and, when the integrated value reaches a certain limit, supplies the switching signal to theswitching circuit 10.

FIG. 2 shows an example of thepower detector 20 in detail. The received signal supplied to aninput terminal 101 from theswitching circuit 10, after being detected by adetector 201, is converted into a received digital signal by an analog-to-digital (A/D)converter 202, and taken out of aterminal 103. Areciprocal calculator 203 calculates the reciprocal of the output of the A/D converter 202, and feeds the result to aterminal 102 as the noise power. This is based on the knowledge that, in a linear modulation process such as amplitude modulation, the noise power is proportional to the reciprocal of the received signal power.

Meanwhile, a structure illustrated in FIG. 3, for example, can be used as a power detector for frequency modulated signals. Referring to the figure, the received signal given to aterminal 101 is supplied to alimiter 204, and then frequency-detected by afrequency discriminator 205. This is a commonly used method in FM communication. Removing the signal component from this output of thefrequency discriminator 205 with afilter 206 gives the noise power. The field strength in this case is obtained as the reciprocal of the noise power. The received signal power and noise power obtained in this manner are outputted from

terminals

103 and 102, respectively.

FIG. 4 is a circuit diagram illustrating theintegrator 30 in detail. Anadder 301 adds the noise power entered from aninput terminal 104 to an integrated value stored in aregister 302. Acomparator 303 produces the switching signal from aterminal 105 when the integrated value from theregister 302 surpasses a predetermined value. The content of theregister 302 is reset by the switching signal.

Next will be explained the operation of the first preferred embodiment shown in FIG. 1 with reference to FIGS. 5A to 5C. FIG. 5A shows the field strength of a prior art diversity receiver, and FIG. 5B, that of a diversity receiver according to the present invention. FIG. 5C shows the output of theregister 302. In the description hereunder, it is supposed for the sake of brevity that electric fields received by antennas vary in a triangular wave shape. In FIGS. 5A and 5B,triangular waves 1 and 2 represent the magnitudes of the fields received from theantennas 1 and 2, respectively, and the thick solid line indicates the field strength actually obtained after diversity, while the broken line shows the field strength of the antenna which is not selected. These conditions will also apply to the waveform diagrams shown in FIGS. 7A and 7B for explaining the operation of the second preferred embodiment to be referred to afterwards.

First will be described the operation of a prior art diversity receiver with reference to FIG. 5A.

It is supposed that the antenna 1 is selected in the initial state. A received signal from the antenna 1 crosses a switching threshold downward at time T_a1, when antenna switching takes place to select theantenna 2. Then at time T_a2, the field strength of theantenna 2 also falls below the switching threshold, and switching takes place to select the antenna 1 again. Since the field strength at the antenna 1 at this time is already below the switching threshold, no more antenna switching takes place until the field strength at the antenna 1 rises and again crosses the switching threshold downward. Thus antenna switching is effective only between T_a1 and T_a2.

Next will be explained the operation of the diversity receiver according to the present invention with reference to FIGS. 5B and 5C.

It is also supposed that the antenna 1 is selected in the initial state. When the content of theregistor 302 reaches a constant value at time Tl, a switching signal is supplied by theintegrator 30 to switch the antenna 1 to theantenna 2 and, at the same time, to reset the content of theregistor 302.

The slope of the curves, shown in FIG. 5C, illustrating integrated values becomes gradually small while the received signal power is rising and, conversely, becomes gradually large as the received signal power falls. At time T2, the integrated value again reaches the constant value, and antenna switching takes place, this time to the antenna 1 whose field strength is smaller. In this case, however, the integrated value from theintegrator 30 quickly rises and reaches the constant value because the field strength is small, so that switching soon takes place again at time T3. In this way, antenna switching takes place at a short interval when the field strength is small, and the better antenna is selected.

FIG. 6 is a block diagram illustrating a second preferred embodiment of the present invention, which is provided with an arrangement for switching the antennas according to a variable threshold. In FIG. 6, the received signal from theantenna switching circuit 10 is entered into thepower detector 20, and at the same time outputted from a terminal 100 to a receiver section (not shown). Acomparator 40 gives a switching signal to anOR gate 60 when the received signal is smaller than the signal level which is set as a threshold in amemory 50A. Meanwhile, a noise power determined by thepower detector 20 is entered into theintegrator 30, which integrates the noise and, when the integrated value reaches a constant value, outputs a switching signal to theOR gate 60. TheOR gate 60 feeds the switchingcircuit 10 with a switching signal from either thecomparator 40 or theintegrator 30. TheOR gate 60 also supplies the switching signal as an updating control signal to thememory 50A which stores the received signal in response to the control signal.

Next will be explained the operation of the second preferred embodiment of the invention with reference to FIGS. 7A to 7C.

FIG. 7A illustrates reception fields for theantennas 1 and 2 and a threshold varying with the antenna switching and is helpful to understand the switching operation. However, in FIG. 7A, the trace of the threshold is simplified in the illustration because of the overlapping with the reception field curves. For this reason, a more precise trace of the threshold is reproduced in FIG. 7C. FIG. 7B shows a curve of integrated values of noise together with a straight line of the constant value.

As stated above, in the second embodiment of the invention, when the field strength is smaller than the threshold stored in thememory 50A, the output of thepower detector circuit 20 is written into thememory 50A as a new threshold. With this structure, when the field strength gradually decreases, the switching threshold correspondingly lowers. However, when the field strength increases, the output of thepower detector 20 is always higher than the threshold stored in thememory 50A, so that no switching signal is generated and the antenna having a greater field strength cannot be selected.

A third preferred embodiment of the invention, illustrated in FIG. 8, is so composed as to solve this problem. In FIG. 8, asecond comparator 70, when the difference between the field strength and the threshold from amemory 50 surpasses a constant value (this constant value is hereinafter called the maximum level difference), provides a memory control signal to thememor 50. Acontroller 80 supplies thememory 50 with memory updating data, which is lower than the output of thepower detector 20 by maximum level difference. Thememory 50, in response to the memory control signal, stores the memory updating data as a new threshold from thecontroller 80.

FIG. 9 illustrates an example of thesecond comparator 70. There are entered the received signal, which is the output of thepower detector 20, and the threshold, which is stored in thememory 50, from

input terminals

107 and 108, respectively. Asubtractor 41 subtracts the threshold from the received signal. Acomparator 42 outputs the memory control signal from a terminal 109 when the balance of subtraction is greater than the maximum level difference.

FIG. 10 illustrates one example of the memory. In the figure, agate circuit 51 will output the updating data fed to a terminal 112 if the memory control signal is supplied to a terminal 111, and will output "0" in all other cases. Agate circuit 52 similarly supplies the output of thepower detector 20 when the switching signal from a terminal 110 is "1". The outputs of the

gate circuits

51 and 52 undergo logical addition by anOR gate 54, and the resultant logical sum is supplied to aregister 55. Therefore, when the memory control signal is "1", the updating data is entered into theregister 55, and when the switching signal is "1", the output of thepower detector 20 is entered into theregister 55. The content of theregister 55 is reloaded by a reloading signal, which has been provided by anOR circuit 53 as the logical sum of the memory control signal and the switching signal. Therefore, as either the memory control signal or the switching signal is entered, the content of the register is reloaded. The content of theregister 55 is outputted as threshold from a terminal 20.

As hitherto stated, according to the present invention, antenna switching is performed on the basis of the noise power and, therefore, is possible regardless of whether the reception field is strong or weak. Furthermore, according to the invention, the switching threshold is varied with changes in field strength to make it possible to always use the antenna having the strong reception field.

Claims

What is claimed is:

1. A receiver for use in switching diversity systems having at least two antennas for reception, comprising:

switching means responsive to an antenna switching signal for switching between said two antennas to produce a received signal;

noise estimating means for producing a first signal indicative of the quantity of noise in the received signal; and

an integrating means adapted to respond to the first signal for integrating the noise to produce a first control signal as said antenna switching signal.

2. A receiver for use in switching diversity systems, as claimed in claim 1, wherein the receiver further comprises power detecting means for producing a second signal indicative of the power of the received signal and wherein the noise detecting means includes means for producing the first signal as the reciprocal of the second signal.

3. A receiver for use in switching diversity systems, as claimed in claim 2, wherein the power detecting means includes means for detecting the power of a linear modulated signal.

4. A receiver for use in switching diversity systems, as claimed in claim 2, wherein the power detecting means includes means for detecting the power of a frequency modulated signal.

5. A receiver for use in switching diversity systems, as claimed in claim 1, wherein the integrating means includes means for adding a plurality of first signals, means for comparing the sum to a predetermined value, and means for supplying the first control signal in accordance with a predetermined relationship between the sum and the predetermined value.

6. A receiver for use in switching diversity systems, as claimed in claim 5, wherein the integrating means includes means responsive to the first control signal for resetting the sum.

7. A receiver for use in switching diversity systems having at least two antennas for reception, comprising:

noise estimating means for producing a first signal indicative of the quantity of noise in the received signal;

an integrating means adapted to respond to the first signal for integrating the noise to produce a first control signal as said antenna switching signal;

power detecting means for producing a second signal indicative of the power of the received signal from said switching means;

memory means adapted to respond to the antenna switching signal for storing said second signal as a threshold signal for antenna switching;

first comparator means for comparing said second signal with the threshold from said memory and for generating a second control signal when said second signal is smaller than the threshold from said memory means; and

gating means responsive to said first and second control signals for producing said antenna switching signal.

8. A receiver for use in switching diversity systems, as claimed in claim 7, further comprising:

second comparator means for comparing said second signal with the threshold from said memory means and for generating a memory control signal when said second signal is greater than the threshold from said memory means by a predetermined value, said memory control signal being applied to said memory as a write enabling signal; and

threshold generating means responsive to said second signal for producing an updated threshold which is smaller than said second signal and is greater than the threshold of said memory means, said updated threshold being supplied from said controlling means to said memory means as said threshold for antenna switching.

9. A receiver for use in switching diversity systems, as claimed in claim 8, wherein the memory means comprises a memory circuit, first gate means responsive to said memory control signal for supplying the updated threshold to the memory circuit, and second gate means responsive to said switching signal for supplying said second signal to the memory circuit.

10. A receiver for use in switching diversity systems, as claimed in claim 9, wherein the memory circuit comprises a register.

11. A receiver for use in switching diversity systems, as claimed in claim 9, wherein the memory means includes means for gating the output of the first and second gate means into the memory circuit and means for gating said memory control signal and said switching signal to produce a memory circuit reloading signal.